1. Field of the Invention
This invention relates to a magneto-resistive head used as a reproducing head in a magnetic recording/reproducing device such as a hard disk drive and VTR.
2. Description of the Related Art
As a method of reproducing a signal recorded on a magnetic recording medium, a method of moving a so-called ring head having a coil relative to a recording medium to induce a voltage in the coil by electromagnetic induction and detecting the induced voltage is widely used. On the other hand, as is disclosed in IEEE MAG-7, 150, (1971), for example, a magneto-resistive head is known as a highly sensitive head for detecting the signal magnetic field on the recording medium. The magneto-resistive head utilizes a phenomenon that the electrical resistance of a certain type of ferromagnetic material varies with the intensity of an external magnetic field.
In recent years, it has been desired to decrease the size of a magnetic recording device and increase the capacity thereof. For this reason, the relative speed of the head to the recording medium is increasingly made lower. This type of magneto-resistive head (which is hereinafter referred to as an MR head) has an advantage that a large reproduced output can be derived irrespective of the relative speed with respect to the recording medium. Therefore, the MR head having the above advantage becomes more important.
The MR head has a magneto-resistive element (film) to which two types of bias magnetic fields are applied. One of the two type of bias magnetic fields is a transversal bias magnetic field which is applied in a direction of an axis of difficult magnetization of the MR element. The transversal bias magnetic field is a bias magnetic field for setting the operating point of the MR element into a linear region in which a detection signal varies in proportion to an external signal and results in a highly sensitive reproduction of a signal.
One example of a method for applying the transversal bias magnetic field is disclosed in Japanese Patent Publication Nos. S53-37205 and S56-40406. That is, there is provided a self-bias method in which a magneto-resistive element and a soft magnetic layer with a non-magnetic layer disposed therebetween are used and a magnetic field created by the sense current is used as the transversal bias. The soft magnetic layer is generally called a soft adjacent layer (SAL). Another example of a method for applying the transversal bias magnetic field is a shunt bias method as is disclosed in Japanese Patent Publication No. S53-25646. Further, another example of a method for applying the transversal bias magnetic field is a method effected by passing a current in the adjacent coil as is disclosed in Japanese Patent Publication No. S53-37206. Still another example of a method for applying the transversal bias magnetic field is a method effected by magnetizing a hard magnetic layer adjacent to the magneto-resistive element as is disclosed in Japanese Patent Publication No. S54-8291.
The other bias magnetic field applied to the MR device is a magnetic field called a longitudinal bias applied in a direction parallel to an axis of easy magnetization of the MR element. The longitudinal bias magnetic field acts to suppress the Barkhausen noise caused by the multi-magnetic domain structure by forming the MR device in the single-magnetic domain structure. Various methods of applying the longitudinal bias are proposed in the prior art. For example, the technique of applying the uniform longitudinal bias to the magneto-resistive element by the exchange coupling between the antiferromagnetic layer and the ferromagnetic layer is disclosed in U.S. Pat. No. 4,103,315. Further, the experiment that the longitudinal bias can be applied to the magneto-resistive element when an FeMn layer is used as the antiferromagnetic layer and a Ni.sub.80 Fe.sub.20 layer is used as the magneto-resistive element is reported in JOURNAL OF APPLIED PHYSICS VOL. 52,2474 (1981). The experiment that the longitudinal bias can be applied to a magnetic field sensitive region of the magneto-resistive element when the FeMn layer is disposed only on the end portion of the magneto-resistive element is reported in IEEE TRANS. MAG-25, 3692 (1989). In both of the above cases, the Barkhausen noise can be suppressed by use of the longitudinal bias magnetic field.
A method effected by using a magnetized ferromagnetic layer in the same manner as in the case of applying the transversal bias is proposed as another method of applying the longitudinal bias. For example, a method of applying a bias to the magneto-resistive element with a thin insulation layer disposed between the magneto-resistive element and the magnetized ferromagnetic layer is proposed in U.S. Pat. No. 3,840,898. In this case, the longitudinal bias, transversal bias and the bias in the intermediate direction can be selectively set by adequately selecting the direction of the magnetization. Further, a method of applying the longitudinal bias by disposing the magnetized CoP layer on the end portion of the magneto-resistive element of the yoke type MR head is introduced in the institution paper MR86-37 from the magnetic recording institution of the Institute of Electronics and Communication Engineers of Japan.
As described above, various methods of applying the longitudinal bias to the MR element are proposed, but when the above methods are applied to a reproducing head for the hard disk drive, the following problems occur.
.gamma.-FeMn, a known material which displays a strong antiferromagnetic property at room temperatures, is exchange-coupled with the magneto-resistive element such as a NiFe layer and is used when the antiferromagnetic layer and the magneto-resistive element are exchange-coupled with each other. For example, as is reported in Japanese Metal Institution (543), Autumn, 1990, the material may give an important influence on the reliability of the device since Mn is liable to be oxidized. When .gamma.-FeMn is formed by sputtering, .alpha.-FnMn may be formed as is pointed out in JOURNAL OF APPLIED PHYSICS VOL 52,2471, (1981). Therefore, it is difficult to obtain stable .gamma.-FeMn as an industrial product.
The magnitude of the longitudinal bias is preferably set to such a value as to cancel the demagnetizing field in the end portion of the magneto-resistive element. If the magnitude of the longitudinal bias is smaller than the magnitude which is necessary to cancel the demagnetizing field, the magneto-resistive element cannot be formed in the single-magnetic domain structure. On the other hand, if the magnitude of the longitudinal bias is larger than the magnitude which is just necessary to cancel the demagnetizing field, the sensitivity of the magneto-resistive element is lowered. The magnitude of the demagnetizing field depends on the shape of the MR element, that is, the track width and film thickness thereof. Therefore, according to the specification of the head, it is necessary to change the magnitude of exchange energy. However, as is disclosed in JOURNAL OF APPLIED PHYSICS VOL. 52,2471 (1981), it is necessary to change the film thickness of the NiFe layer or FeMn layer in order to control the magnitude of the exchange energy between the FeMn layer and NiFe layer. Since the film thickness of the NiFe layer is closely related to the characteristic of the head, the film thickness cannot be freely changed. When the thickness of the FeMn layer is increased, .alpha.-FnMn may be formed.
Thus, it is practically difficult to change the exchange energy between the antiferromagnetic material layer and the magneto-resistive element according to the specification of the head.
Further, as is pointed out in JOURNAL OF APPLIED PHYSICS VOL. 53,2605 (1982), the dependency of the exchange coupling energy between the FeMn layer and the NiFe layer on temperature is large. Therefore, the characteristic of the device may be changed by an influence by heat generation due to the sense current or by the environment of application.
In order to solve the problem which may be caused when the FeMn layer is used as the antiferromagnetic layer, a method of exchange-coupling a TbCo layer with the NiFe layer is introduced in IEEE TRANS. MAG-24,2609 (1988). However, the material is liable to be oxidized and the reliability thereof cannot be maintained for a long period of time even if the environment of application is extremely limited.
A method of applying the longitudinal bias by use of the magnetized ferromagnetic layer is effective when the MR element is disposed in position apart from the recording medium as in the yoke type MR head. However, when the MR element is disposed near the recording medium as in a shield type MR head, there is a possibility that the recording medium may be demagnetized by the leakage magnetic field from the ferromagnetic layer, that is, recorded information may be erased. In order to prevent the recording medium from being demagnetized, the coercive force of the ferromagnetic layer may be reduced. However, in this case, the direction of magnetization of the ferromagnetic layer may be changed by the leakage magnetic field from the recording medium and the longitudinal bias may not be applied.
As described above, the method of applying the longitudinal bias to the magneto-resistive element by the exchange coupling between the antiferromagnetic layer and the ferromagnetic layer has a problem that the long-term reliability cannot be attained since the material of the antiferromagnetic layer generally tends to be oxidized. Further, the method of applying the longitudinal bias by use of the magnetized ferromagnetic layer has no problem when the antiferromagnetic layer is used. However, in a case wherein the MR element is disposed near the recording medium as in the shield type MR head, a problem that magnetic information recorded on the recording medium is erased by the leakage magnetic field from the ferromagnetic layer will occur when a sufficiently large longitudinal bias is applied.